Models of atrial and ventricular arrhythmias will be studied in anesthetized dogs, and in isolated tissue of rabbit and dog hearts. Mathematical models of atrial arrhythmias will also be developed to assist in relating effects of geometry, conduction velocity, and refractory period to self-sustained rapid rhythms, particularly atrial flutter. Models of actual and "pseudo" supernormal conduction in A-V nodal and intraventricular transmission will be tested in the dog heart in situ, and the effect of spontaneous pacemaker activity on intraventricular conduction will be studied. Models of parasystole will be created using a sucrose gap technique in which the cardiac Purkinje fiber in one chamber will be exposed to a medium which enhances pacemaker activity. Modification of the spontaneous frequency by stimuli applied at various times to the quiescent side will be studied. Seasonal variation in the electrophysiological behavior of cardiac tissue will be studied, with special reference to digitalis action.